Zeolites are crystalline aluminosilicate compositions which are microporous and which consist of a negatively charged framework formed from corner sharing AlO2 and SiO2 tetrahedra. The negative framework charge is balanced by cations, which usually reside in the pores. Numerous zeolites, both naturally occurring and synthetically prepared are used in various industrial processes. Zeolites are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent zeolite crystal structure.
Offretite (OFF) and Erionite (ERI) are naturally occurring zeolites which at one time were thought to have the same structure. A single crystal electron diffraction study of the two minerals in Nature, 214, 1005-1006 (1967) found that the a and b axes of OFF and ERI were identical, but the C-axis of the ERI structure was twice as long as that in OFF. The researchers also found that synthetic zeolite T, first disclosed in U.S. Pat. No. 2,958,952, was an intergrowth of these two structures.
There are a number of other references which disclose materials that are intergrowth of OFF and ERI. U.S. Pat. No. 3,699,139 discloses the use of benzytrimethylammonium ion to synthesize an OFF/ERI intergrowth. U.S. Pat. No. 4,086,186 discloses using choline to synthesize ZSM-34 (an intergrowth). M. L. Occelli et al. in Zeolites, Vol. 7, 265-271 (1987) discloses using templates designated DABCO (I) and DABCO (II) to synthesize OFF/ERI intergrowth zeolites. Electron diffraction studies by another author on materials prepared with DABCO (I) and DABCO (II) (using the method of Occelli et al) showed that these were essentially fault-free erionite. See, Studies in Surface Science and Catalysis, 28, 429, Y. Murakami, A. IIjima and J. W. Ward eds., Elsevier, New York, N.Y. 1986. The Si/Al ratio of these erionite materials was stated to be those found in the Occelli reference, which was 5.5. Yet another study reported a TMA-Na—K erionite with Si/Al=4 (Zeolites, (1986) 6, 474-483). Erionite type zeolites were also prepared from highly alkaline solution phase reaction mixtures, attaining a maximum Si/Al ratio of 4.25 in the Na—K-TMA system (Studies in Surface Science and Catalysis (1985) 24, 105-110). Finally, U.S. Pat. No. 4,503,023 discloses zeolites designated LZ-220 which are “more siliceous forms of the prior known mineral erionite and its synthetic analog, zeolite T” (column 19, lines 29-31 of the '023 patent). The Si/Al ratio is stated to be greater than 4. However, as was shown later (see above), zeolite T is an intergrowth of erionite and offretite and the LZ-220 is not pure erionite.
Applicants have succeeded in synthesizing essentially pure erionite with a Si/Al ratio greater than 5.5. The preparation uses an organic template as the major charge balancing cation with alkali or alkaline earth cations being present at sub-stoichiometric levels with respect to Al. Applicants have also been able to modify the crystal morphology to obtain 15-50 nanometer crystals spheroidal in shape versus the needle morphology of both erionite mineral and its known synthetic analog.